Joist span tables for staircase trimmer joist

[Cory H]

Hi all,
I'm converting a loft and have hung 6x2 c24 joists between 2 steel beams (that go from gable to gable). The 6x2s rest on a central wall at first floor level. I wanted to keep the joists as low as possible to maximise headroom.

For the stairs trimming and trimmer joists, I have 2 questions:
1/ Will a 6x4 do the job for both the trimmer and trimming joists? I can find span tables for ordinary joists with standard floor loadings but not for a load from trimmed joists and a stairs.
2/ What is the standard way of fixing the trimmer into the gable block wall? Is it cutting a pocket (like with the steels) or to cut a slot and point a masonry joist hanger into it?
Thanks!

 

[tony1851]

Two 6 x 2s screwed together would be stronger than one 6 x 4.

That layout would be OK, assuming the 4m trimmer is supported off the wall below, as the main floor joists are. If not, it might be subject to excessive deflection in view of the partitions it is supporting, and the point load from the partition at right angles.

In that case, three 6 x 2s screwed together would be better.

Pocketing, or on joist hangers, makes little difference.

 

[George_engineer]

Two 6 x 2s screwed together would be stronger than one 6 x 4.

Not sure I agree with that - should be equally as strong.

OP - the engineer who designed the beams should have done the trimmers really. There are span tables for trimmer openings and it's pretty trivial to look it up if you ask them. Usually they are doubled but I agree with Tony that the trimming joist may need a triple.

 

[tony1851]

Not sure I agree with that - should be equally as strong.

BS 5268;

 

[mikeey84]

BS 5268;

View attachment 287134

Just for my curiosity, does that mean they are 10% stronger?

 

[Dereekoo]

Just for my curiosity, does that mean they are 10% stronger?

Depends which one you are standing on

 

[martygturner]

You can use K9 modification for Trimmers but you need to also factor in the grade of timber you are using and its size , c16,20,22 , 6x2 etc


See https://roymech.org/Useful_Tables/Timber/Timber_design.html

There is a complete guide to working out the suitability of a timber component...but in fairness the rule of thumb is and please do not quote me on it was add a sistered joist to all clear openings...glued and screwed at 100mm opposing centres.

 

[tony1851]

Just for my curiosity, does that mean they are 10% stronger?

It means that the allowable bending stress is 10% greater. But there are other factors to consider, such as excessive deflection, and just because the beam might be 10% 'stronger', it doesn't mean the deflection will be 10% less.
In domestic floor joists, excessive deflection, rather thann excessive stress, is usually the governing criterion.

 

[George_engineer]

BS 5268;

View attachment 287134

Oh I should think that's because the knots etc are assumed to not to coincide. Fair enough.

 

[Dereekoo]

Oh I should think that's because the knots etc are assumed to not to coincide. Fair enough.

Or that the minimum and mean E value of pieces of the same timber differ by 20% or so (and hence the various allowable design stresses) and a one piece section of timber could be down at the minimum E value whilst statistically a two piece section of timber will have an overall value nearer the mean hence the 1.1 K factor. Interestingly floor joist strengths can be enhanced by the same factor but must consist of 4 or more floor joists to be applicable.

 

[Cory H]

Thanks to everyone for the advice.
That makes sense about the 2 parallel timbers. I'm guessing it's similar to the Glulam constructions where the knots are offset and any cracks/separation along the grain will be stopped at the boundary between the timbers.
The trimming joist will sit on the stud wall below - so the maximum span of the trimming and trimmer joists is ~2 metres.
The fact that the point load being imposed by the trimmer on the trimming joist is directly over the central wall below probably makes it safe from a bending standpoint.
@martygturner , you mentioned that (as well as being glued) the sistered joists should be screwed at 100mm opposing centres. Could you describe a bit more about what that means? I'm guessing they are coach screwed? at some interval (is it every 100mm)? alternating towards the top and bottom?